ELECTROPHORETIC MOBILITY STUDY OF DODECYLTRIMETHYLAMMONIUM BROMIDE INAQUEOUS-SOLUTION AND ADSORPTION ON MICROSPHERES

The adsorption of the cationic surfactant dodecyltrimethylammonium bro mide (DTAB) on uniform microspheres (polystyrene latex (PSL) particles ) of various sizes has been studied using electrophoretic light scatte ring, dynamic light scattering, viscosity, and conductivity measuremen ts. The measurements were performed on PSL particles of different diam eters at different surfactant concentrations in an aqueous solution of 0.001 M NaBr. Initially, the uncoated surfactant-free latexes are neg atively charged. After adsorbing a trace amount of the positive DTAB i ons, the electrophoretic mobility of the latexes changes from negative to positive. The electrophoretic mobilities of the DTAB-adsorbed late xes increase with increasing DTAB concentration until reaching a maxim um and then flatten out. The different magnitudes and patterns for the electrophoretic mobilities of the PSL particles of different sizes in dicate that there are different adsorption mechanisms for the adsorbed surfactant on the surfaces of different curvatures. This observation is similar to the previous investigation of the adsorption of block co polymer micelles on PSL particles, where a trend was found in which th e adsorbed layer thickness is inversely proportional to the diameters of the PSL particles. Dynamic light scattering experiments have provid ed useful information on the adsorption only at very low DTAB concentr ations, since the measured hydrodynamic radii are biased at high DTAB concentrations due to the effects from the change in the solution ioni c strength and viscosity. On the basis of this observation, two adsorp tion models are proposed which account for the adsorption of the surfa ctant on large PSL particles and on small PSL particles, respectively.